1. Field of the Invention
The present invention relates to a method and an apparatus for managing a Hyper Frame Number (referred to as an ‘HFN’ hereinafter) for ciphering/deciphering in a mobile communication system. More particularly, the present invention relates to a method and an apparatus for changing an HFN using a Connection Frame Number (referred to as an ‘CFN’ hereinafter) correction value and a CFN offset value generated due to unstable delay in an Iub section between a Radio Network Controller (referred to as an ‘RNC’ hereinafter) and a base station (node B) in a mobile communication system.
2. Description of the Related Art
According to a 3rd Generation Partnership Project (3GPP) standard, ciphering/deciphering for a Radio Link Control (referred to as an ‘RLC’ hereinafter) of a transparent mode is performed at a Media Access Control-dedicated (MAC-d). Ciphering/deciphering functions use a COUNT-C, which is a ciphering sequence number. FIG. 1 is a diagram illustrating a construction of a conventional COUNT-C as suggested by 3GPP TS. As illustrated in FIG. 1, the COUNT-C includes an HFN 101 and a CFN 103.
The HFN forming the COUNT-C is set to a MAC-d of a system and a MAC-d of a terminal when a call is established by the terminal in the system. The HFN increases by 1 whenever one cycle of the CFN elapses. Here, the CFN increases by 1 every 10 ms, and has a value between 0 and 255. A range between 0 and 255 is called one cycle.
After the call is established, when a delay is generated in a section (Iub) between an RNC of a system and a base station (node B), the RNC can receive a time adjustment request from the base station and change a CFN.
However, since the ciphering process performs a ciphering activation time check and manages the HFN value using the CFN, correction of the CFN occurs frequently due to the timing adjustment requests in the system, it is difficult to maintain the same HFNs of the system and the terminal. Here, the ciphering activation time denotes a time point after which the HFN value can be increased according to the cycle of the CFN. That is, before the ciphering activation time, the HFN maintains the substantially same value even when the cycle of the CFN changes. After the ciphering activation time, the HFN can increase when the cycle of the CFN changes. A terminal determines the ciphering activation time with consideration of a margin of about 2000 ms, and informs the RNC of the determined ciphering activation time.
For example, assuming that a CFN of a point at which the RNC receives a ciphering control is 140, a ciphering activation time is 80, and an HFN is 20, when a delay of downlink frame is generated at a point when a CFN is 148 and the downlink frame deviates from a reception window, a base station requests the RNC to perform a time adjustment. Then, the RNC corrects the CFN to 70 according to the request of the base station, and retransmits the downlink frame. At this point, after 100 ms, the correction CFN increases to 80 and becomes the ciphering activation time. Therefore, the RNC increases the HFN to 21 at the next cycle after the current CFN cycle.
As described above, when the CFN is corrected to a previous CFN before the ciphering activation time due to the time adjustment procedure, the HFN is changed. Accordingly, a difference between the HFN of the system and the HFN of the terminal is generated unintentionally.
That is, when an HFN is used as described above, in the case where a delay of an Iub section is not constant and correction of a CFN is frequently generated, a time used for ciphering/deciphering of a specific HFN value may be lengthened or shortened abnormally. For example, as illustrated in FIG. 2, two occurrences of time adjustment procedures are generated due to instability in a delay of the Iub section and a CFN is corrected (201 and 203), so that 78 (205) may be used as an HFN value for an abnormally long time. When this process is repeatedly generated, the HFN of the RNC and the HFN of the terminal do not coincide with each other. Such mismatch of the HFNs may generate a gabbling phenomenon that a user's data is transferred in so fast that a counterpart cannot understand the data. Accordingly, there is a need for an improved apparatus and method for managing an HFN for ciphering/deciphering in a mobile communication system.